For high bit rate, long-haul communications via optical fibers appropriate light sources serving as the transmitter are essential. Semiconductor lasers can be modulated at high speeds and consequently are used extensively in optical transmission systems. As the demand grows for bit rates of 10 Gb/s or more, certain inherent properties of semiconductor lasers come into effect. One of these inherent properties is `chirping` which is a change in the wavelength when the laser is modulated at high bit rates.
The chirping phenomenon in conjunction with the transmission characteristics of a single-mode optical fiber can limit the bit rate times distance product achievable in an optical transmission system. The transmission characteristic of particular importance is fiber dispersion and its effect is dependent on the wavelength of the transmitted signal. Single mode optical fibers are also subject to losses due to absorption. The minimum absorption loss occurs at a wavelength of approximately 1.55 .mu.m whereas the minimum dispersion occurs at a wavelength of approximately 1.3 .mu.m. Inasmuch as the absorption loss has a greater impact on long-haul transmission, lasers which emit in the wavelength range of 1.5-1.6 .mu.m are usually used. The dispersion of the single mode fiber at wavelengths of 1.5 to 1.6 .mu.m results in pulse broadening or intersymbol interference as the digital optical pulse passes through the fiber. Obviously, as the bit rate increases the effects of pulse broadening as a function of transmission distance becomes more acute and ultimately limits the bit rate and/or transmission distance. The dispersion effects are, of course, compounded by the inherent chirping properties of the semiconductor laser which add to the pulse broadening and further limit the bit rate.